Photodetectors typically use a material which absorbs incident light and produces a free electron or electron/hole pair in response to the absorbed light. The number of free electrons depends on the amount of light incident on the material and the efficiency of material used by the photodetector to produce the free electrons.
The detail of any image produced by an array of photodetectors is determined by the accuracy of each photodetector and the total number of photodetectors, usually stated in terms of megapixels. For many applications, the market demands cameras with ever increasing detail in an ever smaller size package. To provide more detail for an image sensor as well as to provide smaller image sensors, the photodetector at each pixel must be made smaller. As each pixel becomes smaller, the number of free electrons that it can produce in a particular interval of time is reduced. The smaller number of free electrons reduces the accuracy of the photodetector and also increases the impact of noise.
Any semiconductor device suffers from leakage current and other effects that cause a small number of free electrons to be continually produced and conveyed through the material. The number of free electrons is small, but they may appear at almost any place in a semiconductor circuit and structure. When these free electrons are mixed with the free electrons from a photodetector, the leakage electrons cause seemingly random variations in the apparent outputs of the photodetectors. When the photodetectors are large and produce a large output, then the noise is small and may be ignored. However, the results from smaller pixels with fewer electron outputs may be significantly affected by the noise electrons.
Photodiode based imagers are the predominant type used for image sensors for large and small camera systems. The pixels in these images have been reduced in size to enable higher resolution cameras for small devices such as cellular telephones, security cameras, and portable cameras. Current ˜1 μm size pixels are near to a limit in terms of manufacturability and ability to produce a signal that is acceptably larger than the noise threshold. Smaller size pixels also are less sensitive to longer wavelengths as the pixel size approaches the wavelength of the light being measured.
Other types of imagers use either quantum dots or organic films as the active element to convert incident light into free electrons. In the quantum dots example, dots are applied to a surface, for example as colloids dispersed in a film. The generated free electrons are then measured using electrodes coupled with circuitry that may be formed in a conventional silicon substrate. The quantum dot film or quantum film may be fabricated so that a pixel converts light into free electron/hole pairs improving sensitivity, especially to longer wavelengths. However, charge collection and read out circuitry associated with the quantum film pixel also produce more noise.